Any Disadvantages To A Self-Biasing Tube Amp?

[evenintheshower]

Having a self-biasing tube amp seems like a good thing, but there aren't a lot of them around. I also don't hear lots of people raving about them.

I am looking at two lower wattage tube bass amps, one is self-biasing (Bearpaw Growler) and the other is not (Old School Amps Model One). Going with a self-biasing circuit seems like the way to go, but are there reasons to avoid them?

[BassmanPaul]

Self biasing a tube output stage cuts the amount of power available as output. The tubes bias voltage is generated by current flow through the tubes cathode resistor which needs to be a high wattage wire wound. The grids are at ground potential. Thus the grids are negative with respect to the cathodes. That cathode voltage reduces the voltage across the tube and it has less voltage to swing through the OPT. The more current that flows through the tube and bias resistor, the higher the cathode voltage and even less for the tube to swing. Most pre-amp tube are run self biased.

Depending on the topology this can cut the output of an amp by something like 50% over what would be available in a fixed bias amp.

Auto bias is different in that the bias is set by a negative voltage as with any fixed bias amp. An electronic circuit monitors the current flow in the cathodes and provides a variable bias to keep the current flow at a preset constant.

Much simplified but I hope this help.

[T-Bird]

Hi.

Self biasing usually means cathode biased output section AFAIK. Nothing fundametally wrong with that approach, but a fixed bias (ie. adjustable, with either a fixed- or variable resistor) is better IMHO.

Are You talking about PP or SE output section?

Edit:Paul beat me to it with far better explanation than I could've given.

Regards
Sam

[evenintheshower]

The two amps I am comparing state that they are single-ended, Class A units.

[BassmanPaul]

Originally Posted by evenintheshower

The two amps I am comparing state that they are single-ended, Class A units.

Single ended amps are almost always self bias although the principles of fixed bias can be applied to them.

[evenintheshower]

Dale Murrow, the owner of Old School Amps, said that he went with an adjustable fixed bias using a trimpot on the turret board to maximize the clean power output of the KT88 used in the amp.

So, Paul, what you have said makes sense to me now. Thank you!

[BbbyBld]

Self biased power amps have kind of a "mushy" feel. They have a bit more compression than a fixed bias amp. Most of the time they are used in conjunction with a tube rectifier. The playing response that these amps usually have is referred to a "push back" because of the compression.

In my experience, they are best used in lower powered guitar amps playing blues. A lot of blues guitar players like and rely on push back to get their sound. That's really the only reason to design a self biased amp because you waste a lot of power in heat through the cathode resistor.

[BassmanPaul]

Originally Posted by evenintheshower

Dale Murrow, the owner of Old School Amps, said that he went with an adjustable fixed bias using a trimpot on the turret board to maximize the clean power output of the KT88 used in the amp.

So, Paul, what you have said makes sense to me now. Thank you!

I have a single ended design using a KT88 that I've owned for more than thirty years. Not liking the waste of the cathode resistor, i removed it and provided a fixed bias supply. Works quite well for more than ten watts output.

Oh, and you are most welcome!

[evenintheshower]

Originally Posted by BbbyBld

In my experience, they are best used in lower powered guitar amps playing blues. A lot of blues guitar players like and rely on push back to get their sound. That's really the only reason to design a self biased amp because you waste a lot of power in heat through the cathode resistor.

Of course, the other reason to design a self-biasing amp is to allow people like me (who are a little bit afraid of electricity) to install new power tubes without having to re-bias the amp or take it to a tech.

[jastacey]

Didn't Traynor have a auto/self biasing feature in the early versions of the YBA-200? I remember they had some issues with it

[seamonkey]

Markbass Classic 300 is a different kind of "self bias"
It uses a micro-controller.
It measures the tubes at start up.
It lets you change the bias from "HiFi" to "Long Life" at the flick of a switch.
If they added an SMPS they'd be the technology leaders.

[evenintheshower]

The Traynor YBA-200 was self-biasing. I thought about getting one, once upon a time. The Markbass looks cool, but it is very expensive.

[aborgman]

Originally Posted by BbbyBld

Self biased power amps have kind of a "mushy" feel. They have a bit more compression than a fixed bias amp.

One need only consider the effect that the signal has on the bias voltage to understand exactly why they compress more.

[acubass]

Is the V4BH as self Bias amp?

[OldHack]

Originally Posted by acubass

Is the V4BH as self Bias amp?

I believe it's actually fixed bias.. Paul??

[BassmanPaul]

Originally Posted by evenintheshower

The Traynor YBA-200 was self-biasing. I thought about getting one, once upon a time. The Markbass looks cool, but it is very expensive.

Not self but auto biasing. the two are different.

[BassmanPaul]

OldHack the V4 series are fixed bias amps though the section of schematic you show are all self biasing. You can see where the bias supply is applied to the grid leak resistors of the outputs.

[BassmanPaul]

Originally Posted by BbbyBld

Self biased power amps have kind of a "mushy" feel. They have a bit more compression than a fixed bias amp. Most of the time they are used in conjunction with a tube rectifier. The playing response that these amps usually have is referred to a "push back" because of the compression.

In my experience, they are best used in lower powered guitar amps playing blues. A lot of blues guitar players like and rely on push back to get their sound. That's really the only reason to design a self biased amp because you waste a lot of power in heat through the cathode resistor.

To add to Bobby's post a little, if a player wants more compression you could try dropping in a different rectifier tube like the 5V4. This one has a fairly high voltage drop across it, 75V IIRC, and will drop the B+ supply a bit. I've done this several times in my own designs for some of my picky customers.

[evenintheshower]

Originally Posted by BassmanPaul

Not self but auto biasing. the two are different.

Yes, auto-biasing. Sorry.

[BassmanPaul]

Originally Posted by evenintheshower

Yes, auto-biasing. Sorry.

No need to apologize but we do have to keep the terms in their proper context or confusion sets in.